This invention relates to electronic thermometers and is suited to most temperature measurement applications. In particular, the invention is applicable to thermometers used in food and dairy processing plants.
Food and dairy processing requires accurate and reliable temperature measurements. Many food and dairy processes are highly dependent on controlled temperatures. For example, pasteurization of dairy products must be conducted at a sufficiently high temperature to destroy particular microorganisms, but not too hot so as to waste energy or destroy the dairy product. If the prescribed temperature is not achieved, then the dairy product may contain salmonella or other harmful bacteria. Similarly, refrigerated processes cool food to inhibit spoilage. The temperature of the refrigerator must be monitored to prevent food spoilage. Accurate thermometers are needed to ensure that the food products are heated or cooled to the proper temperature. Thermometers must accurately indicate the food temperature so that the processing of food and dairy products is conducted safely and properly.
Thermometers are the by far the most common means for monitoring the temperature of foods and dairy products. The operators of processing plants rely on thermometers to accurately display food and dairy temperatures. If a thermometer displays an erroneous temperature, food can spoil without the knowledge of the operator. The spoiled food and dairy products may be unknowingly distributed for human consumption. Because of the danger that spoiled food and dairy products may reach consumers, thermometers used in food and dairy processing plants must be reliable and accurate.
Thermometers for food and dairy processing plants generally are mercury thermometers. Mercury thermometers are reliable and are relatively free of erroneous readings. The U.S. Food and Drug Administration has approved mercury thermometers for use in food and dairy processing. However, when mercury thermometers break, mercury and glass can contaminate the food and dairy products.
While electronic thermometers have long been available, they have not been approved for all food and dairy processing until the present invention. Electronic thermometers can malfunction and display erroneous temperature readings. Their use has been limited to non-critical applications. Examples of electronic thermometers are disclosed in the following references:
______________________________________ U.S. Patent No. Patentee ______________________________________ 4,901,061 Twerdochlib 4,771,393 Ishida 4,586,146 Stillman 4,183,248 West 3,882,728 Wittlinger ______________________________________
This invention provides an electronic thermometer for use in food and dairy processing. The thermometer utilizes a passive trimming circuit in conjunction with high quality resistive temperature devices (RTD) and a modulated current source to produce a linear response over a broad temperature range. The thermometer has several electronic circuits that reduce the likelihood of errors, error detection circuits that shut off the temperature display when an error is detected, and circuits to ensure that rapid temperature changes do not result in false error signals. The combination of these circuits yields a highly reliable and accurate thermometer safe for use in food and dairy processing.
In its preferred embodiment, the current invention comprises an elongated probe housing a pair of redundant RTD temperature sensing elements. Passive trimming circuits allow each RTD to be calibrated to the same reference resistance profile standard. The temperature signals from both sensing circuits are carried by cabling to an instrument housing. The instrument housing contains electronic circuits that process the temperature signals. A pair of redundant measuring circuits each individually interpret the temperature signals and calculate a temperature reading. The display circuit uses dual-slope integration and crystal control for stability. Also, the signal and reference voltages needed to display F.degree. and C.degree. are routed through field effect transistor (FET) switches rather than a mechanical device to eliminate the possibility of error due to contact deterioration.
The measuring circuits are monitored by a differential amplifier that compares the temperature readings of both measuring circuits. If the difference between the two temperature readings exceeds a predetermined threshold level, then the thermometer display becomes blank to prevent the display of erroneous readings. Similarly, the display goes blank when the temperature reading is outside of the range of thermometer.
The differential circuit detects any variations in the temperature readings of the pair of measuring circuits. However, not every variation is indicative of an error. When the thermometer is rapidly adjusting to a new temperature, the transition can cause a temporary imbalance between the pairs of temperature sensing elements and measuring circuits. This temporary imbalance dissipates once the thermometer adjusts to the new temperature. Accordingly, the error detecting differential circuit is disabled and the display does not blank out while the temperature measurement signals change rapidly.
The circuit that inhibits the error signals during rapid temperature changes has failsafe features. If this circuit fails high or fails low, it will not inhibit error signal even during rapid temperature changes. In addition, a test button is provided on the thermometer for simulating a rapid temperature change. Depressing this test button allows the thermometer operator to check that the thermometer and its safeguards are operating properly.
It is an objective of the current invention to provide an electronic thermometer that reliably and accurately displays temperatures. Another objective is to provide a series of safeguards within the thermometer to detect malfunctions and disable the thermometer. An additional objective is to provide a passive temperature probes that are so precise that they are interchangeable and replaceable with no adjustments. After a probe is replaced, the displayed temperature still remains accurate for the entire specified range of the instrument. These and other objectives of this invention are apparent from the description contained within this specification.